1. Field of the Invention.
The field of the present invention relates to overhead cam engines, for use in a variety of applications, such as lawnmowers, generators, pumps, tillers, pressure washers and other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like.
2. Description of the Related Art.
Generally, the intake and exhaust valves of prior known engines are actuated directly by a camshaft located in the cylinder head, or indirectly through the use of rocker arms, tappets, or other similar means. For example, in many existing L-head and overhead valve engines, the crankshaft drives a camshaft which is parallel to the crankshaft and located in the crankcase, and lobes on the camshaft actuate push rods and rocker arms to open and close the valves.
Another arrangement is illustrated in U.S. Pat. No. 5,090,375, assigned to the assignee of the present application, and is hereby incorporated by reference, in which the crankshaft extends externally of the crankcase and the camshaft extends externally of the cylinder head. The camshaft is driven by the crankshaft by means of a belt and pulley, chain and sprocket or the like at one half the rotational speed of the crankshaft. Such speed reduction is typically achieved by mounting a pulley or sprocket on the camshaft that is twice as large as the pulley or sprocket on the crankshaft.
A problem with this and similar designs is that the pulley or sprocket is mounted directly on the portion of the crankshaft which extends from the crankcase, which produces a high force moment on the end of the crankshaft and stress on the crankshaft bearing. To compensate for the stress, the crankshaft bearing must be enlarged and/or the crankcase wall reinforced around the bearing, which increases the cost and size of the crankcase.
An additional problem is that speed reduction is accomplished outside of the crankcase by means of the pulley or sprocket mounted on the camshaft which is twice as large as the pulley or sprocket mounted on the crankshaft. The large pulley or sprocket on the camshaft increases the overall size of the cylinder head and length of the engine.
What is needed is an engine in which the camshaft is not driven directly from the crankshaft, to reduce the stress on the crankshaft bearing and obviate the need for a larger bearing and/or reinforcement of the crankcase wall around the bearing.
A further need is for an engine in which the speed reduction is carried out within the crankcase by the same reduction gearing as in overhead valve and L-head engines, such that a smaller pulley may be to be mounted on the camshaft to reduce the size of the cylinder head and length of the engine and to utilize existing bearings in the crankcase.
A still further need is for an engine in which the above needs are accomplished, yet may be manufactured using existing components and tooling.